Earth-boring bits, such as roller cone rock bits, are employed for drilling oil wells through rock formations, or for drilling blast holes for blasting in mines and construction projects. Earth-boring bits are also referred to as drill bits. During operation, a drill bit is connected to a drill string at one end and typically has a plurality of wear-resistant inserts imbedded in roller cones attached to a bit body at the other end. An insert usually has a substantially cylindrical body portion which is adapted to fit in an insert hole and a top portion which protrudes from the insert hole for contacting an earthen formation.
When a roller cone rock bit is used to drill a borehole, it is important that the diameter or gage of the borehole be maintained at a desired value. The first outermost row of inserts of each roller cone of a rock bit that cuts to a full gage borehole and the corner of borehole is referred to as the gage row. This row of inserts generally is subjected to the greatest breakage and wear as it reams the borehole wall and cuts the corner of the borehole. As the gage row inserts break and/or wear, the diameter of the borehole being drilled may decrease below the original gage diameter of the rock bit. When the bit is worn out and removed, a portion of the hole usually is under-gage. When the next bit is run in the hole, it is therefore necessary to ream that portion of the hole to bring it to the full gage. This not only takes substantial time but also commences wear on the gage row inserts of the newly inserted bit.
In addition to gage row inserts, a conventional bit typically includes a number of inner row inserts located on a roller cone and disposed radially inward from the gage row. These inner row inserts are sized and configured for cutting the bottom of the borehole. Sometimes, a conventional bit also may include a plurality of secondary gage inserts located between the gage row inserts. These inserts, referred to as "nestled gage inserts," typically cut the full gage of the borehole and also assist the gage inserts in cutting the borehole corner. Because a borehole primarily is cut by the collective action of the gage row inserts, nestled gage inserts (if therein), and inner row inserts, they are considered as the main cutting inserts of a rock bit.
In contrast, a conventional rock bit may include a row of heel inserts located on the frustoconical surface of a roller cone. The heel row inserts generally scrape and ream the side wall of a borehole as the roller cone rotates about its rotational axis. As such, the heel row inserts are not considered as the main cutting inserts; rather, they are deemed as auxiliary cutting inserts.
Due to the different functions performed by the main cutting inserts and auxiliary cutting inserts, the two types of inserts experience different loading conditions during use. Thus, their impact of the performance and lifetime of a rock bit is different. Generally, the main cutting inserts have far more significant influence than the auxiliary cutting inserts, and the auxiliary cutting inserts experience less wear and abrasion and breakage than the main cutting inserts.
The performance of a rock bit is measured, in part, by total drilling footage and rate of penetration. As the main cutting inserts on a rock bit wear, the rate of penetration decreases. When the main cutting inserts have been substantially worn out, it is no longer economical to continue drilling with such a rock bit. At this time, the rock bit must be replaced by a new one. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well. Therefore, constant efforts have been made to manufacture main cutting inserts that would increase the rate of penetration and total drilling footage of a rock bit. In particular, there have been numerous attempts to reduce wear and breakage and increase the cutting efficiency of the main cutting inserts.
Two kinds of wear-resistant inserts have been developed for use as main cutting inserts on a rock bit. They include tungsten carbide inserts and polycrystalline diamond enhanced inserts. Tungsten carbide inserts are formed of cemented tungsten carbide. A typical composition for cemented tungsten carbide is tungsten carbide particles dispersed in a cobalt binder matrix. A polycrystalline diamond enhanced insert, an improvement over the tungsten carbide insert, typically includes a cemented tungsten carbide body as a substrate and a layer of polycrystalline diamond ("PCD") directly bonded to the tungsten carbide substrate on the top portion of the insert. Some prior art PCD enhanced inserts utilize polycrystalline diamond substantially over the entire surface of the top portion of a tungsten carbide insert as an improvement over the prior art tungsten carbide inserts. Other prior art PCD enhanced inserts utilize polycrystalline diamond at the central region of the section of the insert that substantially contacts a borehole corner or bottom.
Although the polycrystalline diamond layer is extremely hard and wear resistant, a polycrystalline diamond enhanced insert may still fail during normal operation. The typical failure mode is cracking of the polycrystalline diamond layer due to high contact stress, lack of toughness, and insufficient fatigue strength. A crack in the polycrystalline diamond layer during drilling may cause the polycrystalline diamond layer to spall or delaminate. Furthermore, a crack in the polycrystalline diamond layer may propagate through the cemented tungsten carbide body of the insert and cause complete failure of the insert.
For the foregoing reasons, there exists a need for superhard material enhanced main cutting inserts with increased cutting efficiency to drill through rock formations without substantial breakage or delamination of the polycrystalline diamond layer.